The invention relates to a cathode pot of a cell for producing aluminum by the fused salt reduction process having an outer steel shell, an insulating base layer and on this insulation carbon blocks which enclose iron cathode bars, such that the carbon pot contains the melt of electrolyte and aluminum, and relates too to a process for manufacturing the lining of the pot sidewall.
The fused salt process for producing aluminum by electrolytic reduction of aluminum oxide involves dissolving the latter in a fluoride melt which is made up for the greater part of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell. The surface of the molten aluminum forms the cathode. Dipping into the melt from above are anodes which in conventional processes are made up of amorphous carbon. As a result of electrolytic decomposition of the aluminum oxide, oxygen is formed at the carbon anode with which it reacts to form CO.sub.2 and CO.
The electrolytic process takes place in a temperature range of about 940.degree.-970.degree. C. During the course of the process the electrolyte becomes depleted of aluminum oxide. At a lower concentration of 1-2 wt % aluminum oxide in the electrolyte the anode effect occurs whereby the voltage increases for example from 4-5 V to 30 V and higher. Then at the latest the aluminum oxide concentration must be increased by feeding additional alumina to the cell.
In present day smelter operations the addition of alumina is made almost exclusively by so called point feeding or by central feeding. The previously conventional periodic external feeding for example every 3-6 hours has been replaced by feeding at intervals of only some few minutes. These changes in cell feeding lead to elimination of the protective sidewall layer of solidified electrolyte at the metal level. This layer normally covers the place where the carbon floor blocks meet the sidewalls of the pot and, depending on the form of external feeding is formed by sediments. In the absence of that layer the sidewalls of the pot are therefore exposed more to erosion and corrosive attack by the molten charge in the pot. Consequently the useful service life of the pot is markedly reduced.
The following are the main reasons for the wearing away of the sidewalls of the pot.
Movement of metal and electrolyte which contain abrasive particulate solids, and local turbulence produced by magneto-hydrodynamic effects. PA1 Corrosion of the carbon by the atmosphere produced in the process. PA1 Passage of the direct electric current through the sidewalls. PA1 their inner side is of carbonaceous material and contains a fraction of binder, and PA1 their outer side is of a hard ceramic material which is a poor electrical conductor but a good thermal conductor, is resistant to molten aluminum and the process fumes, and has a coefficient of thermal expansion comparable to that of carbon, both sides being intimately joined and heat can flow almost unhindered from inside to outside. PA1 The electrolyzing d.c. current does not pass through the composite, as the ceramic layer is a poor electrical conductor. PA1 The ceramic layer of the composite is resistant to corrosive attack by the fumes produced in the process. PA1 Any abrasive action of the moving bath and solid particles in it can effect at most the carbon layer; at the latest when the ceramic layer is reached, no further erosion takes place. As a rule, however, pores formed in the carbon layer become filled with solidified electrolyte which prevents further attack. PA1 The aluminum produced is of good smelter quality i.e. the bath does not take up any undesired impurities. PA1 When installing the composite blocks the carbon part can be easily shaped by mechanical means, which for example permits them to be bonded to the carbon elements of the floor.
Proposed in the British Pat. No. 814 038 is to line the walls of the reduction pot with thin ceramic tiles e.g. tiles of a material comprising silicon carbide bonded together with silicon nitride. Tiles of kaolin-bonded silicon carbide and other refractory materials can be employed for the same purpose. Some of the linings made up of such tiles feature a thermally insulating layer e.g. of alumina between the tiles and the sidewall of the steel shell. The floor of the pot is as before fitted with carbon blocks with the gaps between them filled with a rammed mass of non-baked carbon. The disadvantage of these tiles, which mostly contain silicon carbide as the main component, is that the binder used in them is attacked by the molten electrolyte. Also of disadvantage is that the tiles can usually not be bonded close enough to each other to prevent the molten electrolyte penetrating the gaps in time.
Described in the U.S. Pat. No. 3,256,173 is a process for manufacturing the sidewalls of a reduction pot for production of aluminum by the electrolytic fused salt reduction process, in which silicon carbide powder mixed with powdered coke and pitch is employed. The lining of the walls is performed by ramming i.e. compacting this mass into place. The ramming mass described in U.S. Pat. No. 3,256,173 overcomes the disadvantages of preformed ceramic tiles which are bonded together, but it is a poor thermal and d.c. electrical conductor.
The sidewalls of cathode pots made of carbon or silicon carbide feature the following basic properties:
TABLE I ______________________________________ Property Carbon SiC ______________________________________ Thermal conductivity excellent very good Electrical conductivity excellent low Corrosion resistance (gases) moderate good Wear resistance moderate very good Ease of shaping easy difficult Resistance towards liquid Al neutral neutral Resistance towards molten neutral contaminating electrolyte materials ______________________________________